Purpose: To investigate if single energy proton arc therapy (SEPAT) plans generate improved dose to organs at risk (OARs) compared to the Syed interstitial high dose rate brachytherapy (Syed HDR) for a patient with gynecological cancer with lateral disease extension.
Methods: SEPAT delivers proton arc beams by modulating energy at the beam nozzle while maintaining the same incident energy at the gantry during rotation. An index case, high-risk CTV (HRCTV) measuring 76 cc with the lateral dimension of 5.1 cm treated with 14 interstitial needles and 6-cm long tandem in the uterus was selected. Full arc SEPAT plan was designed using 125-157 MeV modulated 157 MeV incident proton beams.
Results: Target coverage of the SEPAT plan was comparable to the Syed HDR plan: 100% of the HRCTV received 90 % of the prescribed dose (V90) for SEPAT, compared to V90 of 90% for Syed HDR. SEPAT robust plan (± 3mm, ±2.5%) generated lower target coverage, 93% dose for V90, which remains to be higher than Syed HDR. D2cc of bladder and rectum were considerably lower for SEPAT plan with 90% (SEPAT) vs 121% (Syed HDR) and 55% (SEPAT) vs 69% (Syed HDR) of prescribed dose, respectively. Other OAR doses such as small bowel and large bowel were also lower with the SEPAT plan than Syed HDR. The SEPAT robust plan demonstrated 98% and 70% of D2cc of bladder and rectum, respectively.
Conclusion: In a case with locally advanced gynecological cancer with lateral disease extension, SEPAT generated a dosimetrically superior plan in comparison to the Syed HDR brachytherapy with interstitial needles. SEPAT may provide a non-invasive alternate to HDR brachytherapy with superior dosimetric profile. Further study is needed with more cases to confirm the result with statistical confidence.